Low-level mixed waste streams are composed of aqueous liquids, heterogeneous debris, inorganic sludge and particulates, organic liquids, and soils. Of particular concern are low-level mixed waste streams that are high in salt content, especially those salt waste streams generated as sludge and solid effluents in nuclear processing applications. For example, the extraction of plutonium and uranium from their ore matrices by the use of strong acids or precipitation techniques produces nitrate salt and heavy metal waste. Chemical compositions typically found in salt waste streams, either high in chloride or high in nitrate, include aluminum trihydroxide, sodium phosphate, MicroCel E (CaSiO.sub.3), water, sodium chloride, trichloroethylene, calcium sulfate, sodium nitrate, and oxides of lead, chromium, mercury, iron, cadmium, and nickel, among other compounds.
Stabilization of salt waste requires that the contaminants and soluble salt anions are effectively immobilized. No single stabilization and solidification technology is known to successfully treat and dispose of salt waste, due to the physical and chemical diversity of salt waste streams. Generally, stabilization refers to the conversion of the waste into a less soluble form, while solidification refers to the micro-encapsulation of the waste in a monolithic solid of high structure integrity. Conventional thermal waste treatment methods, such as incineration or vitrification, are expensive and largely unsuitable for the treatment of salt waste streams because of their reliance on high temperature steps that risk the release of volatile contaminants and the generation of undesirable (e.g., pyrophoric) secondary waste streams. In addition, thermal treatments produce hot spots that affect the quality of a solidified final waste form.
A low-temperature approach is to stabilize hazardous waste by using inorganic (e.g., pozzolanic) binders, such as cement, lime, kiln dust, and/or fly ash. Disadvantages of this approach include a high sensitivity to the presence of impurities, high porosity, and low waste loading volume. Organic binders (e.g., thermosetting polymers) are used even less frequently, because of cost and greater complexity of application. Organic binders are not compatible with water-based wastes, unless the waste is first pre-treated and converted to an emulsion or solid, and organic binders are subject to deterioration from environmental factors, including biological action and exposure to ultraviolet light.
Recently, an alternative non-thermal, low-temperature approach has been developed at Argonne National Laboratory for stabilizing and solidifying low-level mixed waste by incorporating or loading the waste into a phosphate ceramic waste form having a high structural integrity. This technique immobilizes the waste by solidification, such that the waste is physically micro-encapsulated within the dense matrix of the phosphate ceramic waste form, and/or stabilizes the waste by converting the waste into their insoluble phosphate forms. Ceramic encapsulation systems are particularly attractive given that the bonds formed in these systems are ether ionic or covalent, and hence stronger than the hydration bonds in cement systems. Also, the ceramic formulation process is exothermic and economical.
Phosphates are particularly good candidates for stabilization of radioactive and hazardous waste, because phosphates of radio nuclides and hazardous metals are essentially insoluble in groundwater. A salient feature of the low-temperature ceramic phosphate formulation process is an acid-base reaction. For example, magnesium phosphate ceramic waste forms have been produced by reacting magnesium oxide (MgO) with phosphoric acid to form a phosphate of magnesium oxide, Newberyite, as represented in the chemical equation (1), below. EQU MgO+H.sub.3 PO.sub.4 +2H.sub.2 O.fwdarw.MgHPO.sub.4.3H.sub.2 O(1)
The acid-base reaction results in the reaction of the waste components with the acid or acid-phosphates, leading to chemical stabilization of the waste. In addition, encapsulation of the waste in the phosphate ceramic results in physical containment of the waste components. The reaction represented above in Equation (1) occurs rapidly and generates heat, and upon evaporation of the water, a porous ceramic product results.
U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated herein by reference, describes in detail the process steps for setting liquid or solid waste in CBPC products using acid-base reactions. Accordingly, the process involves mixing ground solid waste, including salt waste spiked with heavy metals, with a starter powder of oxide and hydroxide powders of various elements; slowly adding the waste-powder mixture to an acid solution of phosphoric acid or soluble acid phosphates; thoroughly mixing the waste-powder-acid mixture for about a half hour to an hour at ambient temperatures (less than 100.degree. C.), such that the components of the mixture chemically react to form stable phases and a reacted viscous slurry or paste results; and allowing the slurry or paste to set for a few hours into the final CBPC product. Liquid waste is similarly stabilized by mixing the liquid waste with the acid solution (preferably 50:50), and then reacting the waste-acid mixture with the starting powders. The maximum temperature for the process is about 80.degree. C. The CBPC products attain full strength in about three weeks, and exhibit a complex structure, including a major crystalline phase, e.g., Newberyite (MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable phase. The waste components are generally homogeneously distributed within the phosphate ceramic matrix. Unfortunately, however, the porous product (Newberyite) is unsuitable for waste treatment on a large scale.
U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated herein by reference, describes improving the CBPC fabrication process by incorporating two temperature control processes for both reducing heat generation during the encapsulation (reaction) steps and moderating pH conditions (some wastes are unstable at a low pH). The first temperature control process involves pre-treating the phosphoric acid with a carbonate, bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li, Rb) prior to mixing with an oxide or hydroxide powder so as to buffer the acid. In particular, potassium containing alkali compounds (e.g., K.sub.2, KHCO.sub.3, KOH) result in a more crystalline waste form, and the higher the concentration of potassium in the potassium containing compound, the more crystalline the final product, resulting in a higher compression strength, lower porosity, and greater resistance to weathering, compressive forces, and leaching. The second temperature control process involves bypassing the use of the acid and mixing the oxide powder with dihydrogen phosphates of potassium, sodium, lithium, or other monovalent alkali metal, to form a ceramic at a higher pH.
Neutralizing the phosphoric acid solution in equation (1) by adding potassium hydroxide (KOH), as represented in the chemical equation (2) below, reduces the reaction rate and heat generation, and results in the formation of a superior magnesium potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2 O (magnesium potassium phosphate hexahydrate), as represented in chemical equation (3) below. EQU H.sub.3 PO.sub.4 +KOH.fwdarw.KH.sub.2 PO.sub.4.H.sub.2 O (2) EQU MgO+KH.sub.3 PO.sub.4 +5H.sub.2 O.fwdarw.MgKPO.sub.4.6H.sub.2 O(3)
The chemically bonded ceramic phosphate (CBPC) waste form (e.g, MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent durability and a high resistance to fire, chemicals, humidity, and weather. The low-temperature (e.g., room-temperature) process encapsulates chloride and nitrate salts, along with hazardous metals, in magnesium potassium phosphate (MKP) ceramics, with salt waste loadings of up to between approximately 70 weight percent and approximately 80 weight percent. This durable MKP ceramic product has been extensively developed and used in U.S. Department of Energy waste treatment projects.
Phosphates in general are able to bind with hazardous metals in insoluble complexes over a relatively wide pH range and most metal hydroxides have a higher solubility than their corresponding phosphate forms. In addition to the magnesium and magnesium-potassium phosphate waste products discussed above, known waste encapsulating phosphate systems include, but not limited to, phosphates of magnesium-ammonium, magnesium-sodium, aluminum, calcium, iron, zinc, and zirconium (zirconium is preferred for cesium encapsulation). A non-exclusive summary of known phosphate systems and processing details is provided in Table I below, selected according to ready availability of materials and literature about the materials, in addition to low cost.
TABLE I ______________________________________ Phosphate Systems and Processing Details Curing System Starting Materials Solution Time ______________________________________ MKP Ground MgO, ground K Water 1 hour dihydrophosphate crystals Mg phosphate Calcined MgO Phosphoric &gt;8 days acid-water (50/50) Mg--NH.sub.4 phosphate Crushed dibasic NH.sub.4 Water 21 days phosphate crystals mixed w. calcined MgO Mg--Na phosphate Crushed dibasic Na Water 21 days phosphate crystals mixed w. calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric Reacted acid powder, (.apprxeq.60.degree. C.) pressed Zr phosphate Zr(OH).sub.4 Phosphoric 21 days acid ______________________________________
Appropriate oxide powders include, but are not limited to, MgO, Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Zr(OH).sub.4, ZrO, and TiO.sub.2, and combinations thereof. The oxide powders may be pre-treated (e.g., heated, calcined, washed) for better reactions with the acids. While no pressure is typically applied to the reacted paste, about 1,000 to 2,000 pounds per square inch may be applied when zirconium-based powders are used.
The acid component may be dilute or concentrated phosphoric acid or acid phosphate solutions, such as dibasic or tribasic sodium, potassium, or aluminum phosphates, and the paste-setting reactions are controllable either by the addition of boric acid to reduce the reaction rate, or by adding powder to the acid while concomitantly controlling the temperature.
Representative bulk constituents for salt waste include, but are not limited to, activated carbon, Na.sub.2 (CO.sub.3).sub.2, widely used cation or anion exchange resins, water, NaCl, Na(NO.sub.3).sub.2, Na.sub.3 PO.sub.4, and Na.sub.2 SO.sub.4. The salt waste may be reacted with phosphoric acid to any consume carbon dioxide (CO.sub.2) present, prior to mixing the salt waste with the oxide powders or binding powders, as the evolution of CO.sub.2 results in very porous final ceramic products.
Unfortunately, however, encapsulation of low-level mixed waste into CBPC products is currently of limited practical use for waste that is predominantly comprised of salts, such as chlorides, nitrates, and sulfates. Efforts to encapsulate salt waste in phosphate ceramic products are hampered by low maximum waste loading capacities, because of interference of the salt anions with ceramic-setting reactions, leaching of soluble salt anions from the resulting highly porous ceramic product (especially in aqueous environments), and rapid structural degradation of the ceramic product caused by the high leach rates. Also, environmental stresses degrade the integrity of known CBPC waste forms over time. For example, exposure to repeated cycles of wetting, drying and/or freezing, or acidic or other conditions conducive to leaching may affect the long term effectiveness of waste encapsulated CBPC waste forms.
A need in the art exists for a method for disposing of salt waste that involves a low-temperature stabilization process and improves resistance to leaching, without degrading the integrity of the ceramic phosphate product.
The present invention is a process and product for safely containing radioactive and/or hazardous waste comprised of salt anions in a phosphate ceramic product, involving a new and surprisingly effective immobilization technique. The invented process and product involves the application of a specific polymer coating to the exterior surface of a phosphate ceramic composite encapsulating waste, such that the polymer coating infiltrates the surface structure and adheres to and/or bonds to the phosphate ceramic composite matrix, effectively isolating the waste from the environment and improving the leach resistance of the phosphate ceramic composite. The polymer coating contains at least one inorganic metal compound, preferably an inorganic metal oxide of magnesium or silicon.
Therefore, in view of the above, a basic object of the present invention is to provide an improved process and product for immobilizing hazardous, radioactive, and/or mixed salt waste in phosphate ceramic composites.
Another object of the invention is to provide a safe, low temperature, economical process and product for immobilizing salt waste in a phosphate ceramic product that increases the loading capacity and improves the leach resistance of the salt waste within the phosphate ceramic product.
A further object of the invention is to provide process and product for immobilizing large volumes of salt waste in a durable, long term storage phosphate ceramic product.
Additional objects, advantages, and novel features of the invention are set forth in the description below and/or will become apparent to those skilled in the art upon examination of the description below and/or by practice of the invention. The objects, advantages, and novel features of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.